17.11 Implementing the Varargs Macros

GCC comes with an implementation of <varargs.h> and
<stdarg.h> that work without change on machines that pass arguments
on the stack. Other machines require their own implementations of
varargs, and the two machine independent header files must have
conditionals to include it.

ISO <stdarg.h> differs from traditional <varargs.h> mainly in
the calling convention for va_start. The traditional
implementation takes just one argument, which is the variable in which
to store the argument pointer. The ISO implementation of
va_start takes an additional second argument. The user is
supposed to write the last named argument of the function here.

However, va_start should not use this argument. The way to find
the end of the named arguments is with the built-in functions described
below.

— Macro: __builtin_saveregs ()

Use this built-in function to save the argument registers in memory so
that the varargs mechanism can access them. Both ISO and traditional
versions of va_start must use __builtin_saveregs, unless
you use TARGET_SETUP_INCOMING_VARARGS (see below) instead.

On some machines, __builtin_saveregs is open-coded under the
control of the target hook TARGET_EXPAND_BUILTIN_SAVEREGS. On
other machines, it calls a routine written in assembler language,
found in libgcc2.c.

Code generated for the call to __builtin_saveregs appears at the
beginning of the function, as opposed to where the call to
__builtin_saveregs is written, regardless of what the code is.
This is because the registers must be saved before the function starts
to use them for its own purposes.

— Macro: __builtin_args_info (category)

Use this built-in function to find the first anonymous arguments in
registers.

In general, a machine may have several categories of registers used for
arguments, each for a particular category of data types. (For example,
on some machines, floating-point registers are used for floating-point
arguments while other arguments are passed in the general registers.)
To make non-varargs functions use the proper calling convention, you
have defined the CUMULATIVE_ARGS data type to record how many
registers in each category have been used so far

__builtin_args_info accesses the same data structure of type
CUMULATIVE_ARGS after the ordinary argument layout is finished
with it, with category specifying which word to access. Thus, the
value indicates the first unused register in a given category.

Normally, you would use __builtin_args_info in the implementation
of va_start, accessing each category just once and storing the
value in the va_list object. This is because va_list will
have to update the values, and there is no way to alter the
values accessed by __builtin_args_info.

— Macro: __builtin_next_arg (lastarg)

This is the equivalent of __builtin_args_info, for stack
arguments. It returns the address of the first anonymous stack
argument, as type void *. If ARGS_GROW_DOWNWARD, it
returns the address of the location above the first anonymous stack
argument. Use it in va_start to initialize the pointer for
fetching arguments from the stack. Also use it in va_start to
verify that the second parameter lastarg is the last named argument
of the current function.

— Macro: __builtin_classify_type (object)

Since each machine has its own conventions for which data types are
passed in which kind of register, your implementation of va_arg
has to embody these conventions. The easiest way to categorize the
specified data type is to use __builtin_classify_type together
with sizeof and __alignof__.

__builtin_classify_type ignores the value of object,
considering only its data type. It returns an integer describing what
kind of type that is—integer, floating, pointer, structure, and so on.

The file typeclass.h defines an enumeration that you can use to
interpret the values of __builtin_classify_type.

These machine description macros help implement varargs:

— Target Hook: rtx TARGET_EXPAND_BUILTIN_SAVEREGS (void)

If defined, this hook produces the machine-specific code for a call to
__builtin_saveregs. This code will be moved to the very
beginning of the function, before any parameter access are made. The
return value of this function should be an RTX that contains the value
to use as the return of __builtin_saveregs.

This target hook offers an alternative to using
__builtin_saveregs and defining the hook
TARGET_EXPAND_BUILTIN_SAVEREGS. Use it to store the anonymous
register arguments into the stack so that all the arguments appear to
have been passed consecutively on the stack. Once this is done, you can
use the standard implementation of varargs that works for machines that
pass all their arguments on the stack.

The argument args_so_far points to the CUMULATIVE_ARGS data
structure, containing the values that are obtained after processing the
named arguments. The arguments mode and type describe the
last named argument—its machine mode and its data type as a tree node.

The target hook should do two things: first, push onto the stack all the
argument registers not used for the named arguments, and second,
store the size of the data thus pushed into the int-valued
variable pointed to by pretend_args_size. The value that you
store here will serve as additional offset for setting up the stack
frame.

Because you must generate code to push the anonymous arguments at
compile time without knowing their data types,
TARGET_SETUP_INCOMING_VARARGS is only useful on machines that
have just a single category of argument register and use it uniformly
for all data types.

If the argument second_time is nonzero, it means that the
arguments of the function are being analyzed for the second time. This
happens for an inline function, which is not actually compiled until the
end of the source file. The hook TARGET_SETUP_INCOMING_VARARGS should
not generate any instructions in this case.

Define this hook to return true if the location where a function
argument is passed depends on whether or not it is a named argument.

This hook controls how the named argument to FUNCTION_ARG
is set for varargs and stdarg functions. If this hook returns
true, the named argument is always true for named
arguments, and false for unnamed arguments. If it returns false,
but TARGET_PRETEND_OUTGOING_VARARGS_NAMED returns true,
then all arguments are treated as named. Otherwise, all named arguments
except the last are treated as named.

You need not define this hook if it always returns zero.

— Target Hook: bool TARGET_PRETEND_OUTGOING_VARARGS_NAMED

If you need to conditionally change ABIs so that one works with
TARGET_SETUP_INCOMING_VARARGS, but the other works like neither
TARGET_SETUP_INCOMING_VARARGS nor TARGET_STRICT_ARGUMENT_NAMING was
defined, then define this hook to return true if
TARGET_SETUP_INCOMING_VARARGS is used, false otherwise.
Otherwise, you should not define this hook.